Various metal ions are essential elements in plant nutrition and soil deficiency thereof is very often the cause of poor plant growth. Of particular importance for plant growth are iron, manganese, copper and zinc ions, iron being especially important. The following description of the prior art will focus in particular on the problems associated with the iron administration, but it should be understood that similar problems are associated also with the administration of the other three metals mentioned before.
Iron ions in the soil are present either in their di--(Fe.sup.+2) or trivalent (Fe.sup.+3) form, neutral or alkaline conditions favouring the latter. In their trivalent form iron ions hydrolyse water to form highly stable precipitates, the solubility coefficient being in the order of 10.sup.38. Thus, in soils having a neutral or basic pH, such as calcareous soils in which the calcium carbonate acts as a buffer maintaining a high pH, iron precipitates rapidly becoming unavailable to plants' roots.
It has been hitherto proposed to administer iron in a divalent ionic salt form. However, this method did not prove to be sufficiently efficient.
The most widely accepted method for the administration of iron is in the form of a complex of a synthetic chelate, in particular FeEDDHA or FeEDDHMA (Chen and Barak, 1982, Advances in Agronomy, 35:217-239). The main disadvantage of these synthetic chelates is in their high price (see, for example, Barak and Chen, 1987, Soil Sci. Soc. Am. J. 51:893-896) but, as noted by Barak and Chen and others, the use of FeEDDHA to the administration of iron to the soil was nevertheless the method of choice. The high efficiency of using the synthetic chelate EDDHA for the administration of iron has also been shown by Bell, 1977 (Principles and Application of Metal Chelation, Clarendon Press, Oxford).
The use of organic acids, such as citrate, as chelating agents of metal ions has been proposed but as has been shown (e.g. Bell, 1977, supra), citrate was found to be far inferior in its properties than the above synthetic chelating agents at pH below 11 (see page 133, fourth paragraph of Bell, 1977, supra). As shown by Cline et al., 1982 (Soil Sci. Soc. Am. J. 46:1158-1164), ion chelates comprising citrate as the chelating agent were unstable at pH above 7.
A composition comprising citric acid and metal ions has been described in SU-886874. The composition according to this patent is probably a pesticidal composition intended for topical application onto plant leaves and comprises various other ingredients including an adherent substance and sulfur which latter is probably the pesticidally active agent. A compositon comprising ferric ions and citric acid has been described in CA-1,152,769. The composition according to this latter patent comprises also a nitrogen source which is an essential ingredient, and is used for the topical administration of ferric ions to plants, via their leaves. None of these patents discloses or even suggests that citric acid may be used as a chelating agent in composition for administering metal ions to plant roots.
The prior art thus taught away from using organic acids in general, and citrate in particular, as chelating agents for the administration of metal ions to the soil in a form available to plant roots. The entire research was concentrated on finding more efficient ways of using synthetic chelates to this end.